1. Field of the Invention
The present invention relates to a process for the preparation of a ferrite magnetic particulate for magnetic recording, and more particularly relates to a finely divided ferrite magnetic material, which is suitable for use in perpendicular magnetic recording system.
2. Description of Prior Arts
Heretofore, there has been used a magnetic recording system wherein a recording medium such as magnetic recording tape is magnetized along its longitudinal direction in the surface area thereof. A perpendicular magnetic recording system has been recently proposed for accomplishing high density magnetic recording, and various magnetic mediums for use in this system are under study.
There are known methods wherein a magnetic material layer is formed on a substrate (i.e., support) such as a film by a sputtering process, a vacuum deposition process, etc. as the method of preparing the magnetic recording medium for use in the perpendicular magnetic recording system. For example, there has been developed a magnetic recording medium wherein a layer of a magnetic material such as cobalt-chromium is formed on a substrate by the sputtering process.
However, the process for preparing the magnetic recording medium utilizing said sputtering process or vacuum deposition process has disadvantages in productivity and product quality as compared with conventional process for preparing the magnetic recording medium utilizing the conventional coating process. Accordingly, methods utilizing the coating process are also studied as the process for preparing the magnetic recording medium for use in the perpendicular magnetic recording system.
For example, there has been proposed a process for preparing the magnetic recording medium for use in the perpendicular magnetic recording system wherein a hexagonal ferrite such as hexagonal barium ferrite in the form of fine particles having a hexagonal plate shape is used as magnetic material, the hexagonal ferrite is mixed with and dispersed in a resin (binder) and the surface of a substrate is coated with the ferrite.
As typical processes for preparing the hexagonal ferrite such as hexagonal barium ferrite used as the magnetic particulate for the above-mentioned magnetic recording medium, there are known wet processes (such as a co-precipitation process and a hydrothermal synthetic process), a vitrification process, etc.
The present invention provides an improved process for the preparation of hexagonal ferrite magnetic particulate by the vitrification process.
The preparation of the hexagonal ferrite magnetic material by the vitrification process is generally carried out in the process comprising steps of:
(1) melting a starting material mixture containing a basic component for hexagonal ferrite, a coercive force-reducing component and a glass-forming component,
(2) rapidly cooling the resulting molten mixture to obtain an amorphous material,
(3) heat-treating said amorphous material to form ferrite crystals, and
(4) removing components other than the ferrite crystals from the material resulting from said heat-treatment.
In summary, the process comprises the following steps. A mixture of starting materials containing the desired ferrite component and a glass-forming component is melted and then rapidly cooled to form an amorphous material which is then heat-treated as such to form and deposit hexagonal ferrite crystals therefrom, and other materials such as glass component, etc. than the ferrite crystals are removed from the material obtained from the above heat treatment (hereinafter referred to as heat-treated material).
The ferrite magnetic material in the shape of hexagonal plate, which is suitable for use in the perpendicular magnetic recording system, is in the form of fine particle wherein the diameter of the hexagonal plate is 0.1 .mu.m or below and the thickness thereof is 0.03 .mu.m or below. However, the conventional vitrification process is liable to produce ferrite magnetic particulate in the form of agglomerated mass because of presence of a small amount of substances other than the ferrite crystals. Accordingly, the satisfactorily dispersible ferrite fine particulate is hardly obtained in a high yield by the vitrification process.
For the reasons described above, Japanese Patent Provisional Publication No. 57(1982)-56328 discloses a process including, after the aforementioned fourth step, namely, the process for removing components other than the ferrite crystals from the materials resulting from the heat-treatment, a step for pulverizing the obtained particles (including particles in the form of agglomerated mass) in a wet pulverizer, whereby enhancing the dispersibility of the obtained ferrite particulate.
As a result of study of the present inventor, however, it has been noted that the pulverization of the particles after removal of the glass component is liable to damage the ferrite crystals even in the wet pulverizing process. Further, even if the damage of the ferrite crystals is brought about partly, there is introduced into the ferrite magnetic particulate a practically unfavorable decrease of magnetic property such as reduction of saturation magnetization and residual magnetization, and elevation of coercive force. More in detail, a magnetic recording medium containing such magnetic particulate as having reduced saturation and residual susceptibilities shows decrease of saturation magnetic flux as well as residual magnetic flux. Such decrease of the magnetic flux is unfavorable, because it brings about decrease of strength of an electric signal provided thereby. The elevation of coercive force in the magnetic particulate unfavorably increase the coercive force of the magnetic recording medium.
It may be assumed that the damage of ferrite crystals can be avoided by employing a shortened pulverization period or a mild pulverizing condition. However, these processes are not effective to satisfactorily divide the agglomerated mass into single particles, and accordingly ot a small amount of agglomerated mass remains in the resulting ferrite magnetic particulate material. The magnetic material containing not a small amount of the agglomerated mass is hardly dispersed in a binder uniformly, resulting in a heterogeneous coating dispersion. The heterogeneous dispersion is hardly coated on a substrate uniformly, and thus formed magnetic material-containing layer is made ununiform, resulting in decrease of various properties of the magnetic recording medium.